1. Field of the Invention
The present invention relates to a fixing unit using a fixing belt for use in an image forming apparatus such as a color laser printer.
2. Description of Related Art
Japanese Patent Application Laid-open No. 2003-156966 proposes a fixing apparatus (belt fixing unit) as shown in FIG. 16. This fixing apparatus comprises: a pair of rollers, namely a fixing roller 102 and a pressing roller 103, that are provided midway along a so-called horizontal transport passage, that is, a transport passage through which paper having a toner image formed thereon is transported approximately in the horizontal direction; a heating roller 104 that is provided approximately vertically above the fixing roller 102 and that incorporates a heater 105 for heating the fixing roller 102; a fixing belt 101 that is formed as an endless belt wound around and between the fixing roller 102 and the heating roller 104. Here, for the control of the fixing temperature, a non-contact temperature sensor 106 is provided to face the fixing roller 102 from the upstream side thereof with respect to the paper transport direction, with the fixing belt 101 passing therebetween.
In recent years, in comparatively small-size image forming apparatuses such as color laser printers, for further space saving, such models have been becoming increasingly popular as are designed to have smaller footprints by adopting a so-called vertical transport passage (see FIG. 1), that is, a transport passage through which paper having a toner image formed therein is transported from bottom to top approximately in the vertical direction rather than approximately in the horizontal direction.
However, if the arrangement of the temperature sensor (provided to face the fixing roller 102 from the upstream side thereof with respect to the paper transport direction, with the fixing belt 101 passing therebetween) in the belt fixing unit proposed in Japanese Patent Application Laid-open No. 2003-156966 mentioned above is adopted intact in a vertical-transport belt fixing unit (doing so is equivalent to rotating FIG. 16 through 90 degrees clockwise), the sensing surface of the temperature sensor points upward approximately in the vertical direction. This makes the sensing surface prone be soiled with toner, paper dust, and the like falling from the upper part of the transport passage, possibly degrading the sensing accuracy of the temperature sensor.